This invention relates to the treatment of gases to remove pollutants, and has particular relationship to the removal of nitrogen oxides from gases containing these oxides to prevent the emission of these oxides to the atmosphere. Nitrogen oxides are known atmospheric pollutants and are believed responsible for the production of photochemical smog and "acid rain" phenomena. Although there are many chemically derivable oxides of nitrogen, the principal oxides implicated in atmospheric pollution are mixtures of the stable oxides, nitric oxide, NO, and nitrogen dioxide, NO.sub.2, and their equilibrium compounds. These nitrogen oxides will be referred to herein as "NO.sub.x ".
Very high concentrations of NO.sub.x may be evolved in processes utilizing concentrated nitric acid in the chemical milling and passivation of stainless steels, tungsten, molybdenum, copper and the like, and the dissolution of precious metals. Lower concentrations of NO.sub.x are present in tail-gases from nitric acid manufacture (0.1 to 0.5% by volume) and in combustion flue gases from industrial furnaces or internal combustion engines. Various types of treatment processes have been proposed which involve either catalytic conversion or catalytic reduction of the nitrogen oxides, or wet scrubbing with various solutions of metal complexes or metallo-organic compounds. Catalytic methods of NO.sub.x removal generally require the use of elevated gas temperatures and the addition of a reducing agent such as NH.sub.3, CO, H.sub.2 or hydrocarbons. Catalytic NO.sub.x reduction methods become very expensive if the NO.sub.x concentrations are higher than normal flue gas NO.sub.x concentrations, or if the bulk gas must be heated from near ambient conditions to the elevated temperatures required for catalytic reduction. Therefore, catalytic NO.sub.x reduction processes are not normally applicable to processes generating high NO.sub.x concentrations in the exhaust gases; i.e., concentrations higher than 0.5% by volume (5,000 parts per million), or where the exhaust gases are at or near atmospheric or ambient temperatures.
Wet processes proposed for NO.sub.x absorption include various forms of aqueous alkaline solutions, transition metal complexes, and reductive reagents, such as catalyzed sulfite solutions, or oxidative reagents, such as permanganates. Processes employing transition metal complexes.sup.(1) and/or organic-compound-containing salts or reductive/oxidative chemicals, generate liquid waste byproducts which are themselves biotoxic water pollutants and cannot be sewered or easily disposed of. Wet process treatment with alkaline-earth oxides, hydroxides and carbonates, particularly calcium carbonates, has also been proposed, but these treatments have very unfavorable overall absorption kinetics for NO.sub.x and are ineffective unless very large and uneconomic sizes of equipment are employed. FNT .sup.(1) See Hishinuma U.S. Pat. No. 4,081,509 and Saitoh U.S. Pat. No. 4,087,372.
It is accordingly an object of this invention to overcome the disadvantages and drawbacks of the prior art and to remove nitrogen oxides from a gas effectively and at rapid, economic absorption rates, without the creation of by-product waste liquor pollution problems. It is an object of this invention to provide a method and apparatus for carrying out such removal.